The most enduring image of Colin Firth is still as Mr Darcy, wet blouse dripping, emerging from a lake. But come next March, that image could be knocked off the top spot by another: Firth beaming, on stage at the Kodak theatre in LA, clutching a golden statue.

Almost out of nowhere, Firth has emerged from the Venice-Toronto festival doubleheader as a serious contender for an Oscar, just as Mickey Rourke did last year. Firth won the best actor prize at Venice for his role as a grieving gay professor in Tom Ford's debut A Single Man. Then the film was the subject of a bidding war among US distributors in Toronto, won by that ferocious Oscar hound Harvey Weinstein, who pushed Kate Winslet to the podium last year.

We all know that Oscar loves Brits, particularly posh, self-deprecating ones; and it loves gay guys, so long as they are suitably tragic. Morgan Freeman as Nelson Mandela in Invictus still looks like the race is his to lose (even though no one has seen it yet), and Clooney, Damon, Day-Lewis and Duvall will be heavyweight competition. But if Weinstein can get Firth to tout himself around the chatshow sofas and the Hollywood tea parties, it could be a close thing.

So why does the idea of Firth as this year's Winslet or Helen Mirren, a British national treasure as Oscar frontrunner, seem so incongruous? He's always been a fine actor, sometimes better than that. But unlike Winslet or Mirren, he has never been awards bait. His mantelpiece is bare, apart from a European Film Academy audience prize for Bridget Jones' Diary back in 2001. The public likes him, which is why he gets cast in virtually every Brit pic with commercial ambitions. But his career lacks gravitas.

Since his breakthrough as the ultimate thinking woman's crumpet in Pride and Prejudice, Firth has spent most of his time subverting his brooding Mr Darcy image by horsing around with amiable skill in a string of often mediocre, but sometimes very successful British comedies.

He sent himself up as Mark Darcy in the Bridget movies. He's a regular trouper for the revived Ealing Studios, which landed him in critically-derided fare such as Dorian Gray. When Mamma Mia! producer Judy Craymer came to casting, she knew exactly who her middle-aged female fanbase wanted to see as Meryl Streep's old lovers: "James Bond and Mr Darcy, who else?" According to Craymer, Firth loved the fact that his repressed English Romeo ended up in arms of a Greek boy, although she had to cut those scenes short for fear of upsetting her audience too much.

Firth was always a reluctant sex symbol, and an ambivalent star who sometimes had to be talked into playing the frivolous media game by his producers. He certainly comes across as someone who thinks there are more important things in life than prancing around in slap and spangles. Despite his flair for comedy, Firth is a serious fellow. At next month's London film festival, he will launch his project Brightwide (a website dedicated to political cinema) and host a panel to discuss how politically-engaged directors can change the world. Yet his own weightier performances in films such as Michael Winterbottom's Genova have gone largely unseen.

"I happen to think he's the finest actor of his generation," says Barnaby Thompson, the head of Ealing Studios who also directed him in two St Trinian's films. "He moves between drama and comedy, which some find confusing. The people who get nominated for things tend to be earnest 24/7. But when you see something like Genova, he's fantastic, so he was always going to have his moment when one of those serious things sparked."

Fashion designer Tom Ford's directorial debut, an adaptation of Christopher Isherwood's A Single Man, is not just moving, with brilliant acting from a fine central cast. It's also startlingly beautiful